1. Field of the Invention
The present invention relates to a fiber optic sensor for sensing vibration of a structure, and more particularly, to a patch-type extrinsic Fabry-Perot interferometric fiber optic sensor in which the existing EFPI (Extrinsic Fabry-Perot Interferometer) fiber optic sensor and a direction-detecting sensor, which is usable as an actuator as well as senses strain of the structure, are combined, and a real-time structural vibration monitoring method using the fiber optic sensor.
2. Background of the Related Art
Recently, many researches have been made for a smart structure which can prevent damage of the structure by sensing and suppressing vibration of the structure and thus reduce the cost of maintenance and repair of the structure.
Such a smart structure includes a sensor system for sensing the variation of an external environment, a brain system for processing sensed information, and an actuator system for actively copying with the sensed variation of the external environment. The brain system comprises a microprocessor which performs a signal process and has a built-in database for characteristics of the structure. In the actuator system, piezoelectric ceramics, ER (Electro-Rheological) fluid or MR (Magneto-Rheological) fluid, which is controllable fluid, and functional materials such as shape memory alloys, may be used.
In the sensor system, semiconductor sensors, metal film sensors, piezoelectric sensors, fiber optic sensors, etc., may be used. In the case of constructing the sensor system using the fiber optic sensors, the sensor system is not affected by electromagnetic waves, and has a very wide operating temperature range. Also, since the optical fiber has a very fine diameter and is flexible, a user can easily construct a sensor of a desired size. Also, the optical fiber can provide a high resolution.
The fiber optic sensor implements a method using transfer/non-transfer of light according to a wave end of an optical fiber, a method using polarization of light, and a method using a light interference such as Mach-Zehnder, Michelson or Fabry-Perot interferometric fiber optic sensor, etc. Among them, the fiber optic sensor implementing the method using the light interference (i.e., interference type fiber optic sensor) measures a ratio of strain of a structure from an interference signal due to a difference of light paths.
However, although the interference type fiber optic sensor, as shown in FIG. 9, can accurately sense the ratio of strain of the structure when the light intensity is in a linear section, it presents a distorted signal with respect to the strain which is out of the linear section having a small width.
As shown in the drawing, relatively good intensity output signal, I1 can be extracted in linear section, S1, while distorted signal, I2 can be produced in a non-linear section, S2, which has different initial optical phase. And the distortion of output signal becomes severe when the amplitude of dynamic strain is excessively large; which is described as I3 and S3 in FIG. 9.
Also, a structural vibration of the structure may cause signal distortion, and this causes the use of the interference type fiber optic sensor as a vibration sensor to be restrictive.
In order to solve this, a cantilever extrinsic Fabry-Perot interferometer sensor and a quadrature phase shifted fiber optic sensor have been developed. However, it is difficult for these fiber optic sensors to perform a real-time signal process and to measure the range of strain.
Meanwhile, many researches have also been made for a smart sensor technology in which one sensor or actuator performs both functions of the sensor and the actuator, instead of the smart structure composed of three parts, i.e., sensor system, brain system and actuator system.
This type of a sensoriactuator can heighten the stability in control, the structural stability when it is inserted into or attached to the structure, and the spatial efficiency. Also, it is very economical since the sensor and the actuator can be replaced by one sensoriactuator. In the early stage, only researches for a simple combination of the sensor and the actuator were made, and in the 1990's, many types of sensoriactuators using piezoelectric materials were developed.
However, the performance of the sensoriactuator as a sensor or an actuator deteriorated due to the non-linear behavior or hysteresis behavior of the piezoelectric material, the uppermost limit of a high voltage caused by the use of a compensation circuit, etc., and this caused the continuous related researches not to be made.
FIG. 10 illustrates the frequency characteristics of a sensoriactuator using the existing piezoelectric material and a self-sensing bridge circuit only, in which (a), (b) and (c) show the results of measurement with resistance ratios used in the circuit varied.
As shown in the drawing, although the phase difference of the frequencies should be kept constant in order that the actuator is used as the sensor, the magnitude actually increases even in the case that the phase is kept constant, and this causes only the frequency component of a specified mode of the structure to be emphasized to limit the range of a usable frequency band.